Chapter 4 - Elements of Design

4.2. Horizontal Alignment

4.2.4. Lane Width Transitions and Shifts

Lane width transitions can occur at several locations including:

• Lane width transitions which are to be developed for curves (see Section 4.2.3. Pavement Widening on Curves)

• Connections to existing pavement – such as pavement tapers which occur at the back of a turnout on an existing side road

• Transitions to a wider lane – such as a truck lane or a one-way, one-lane ramp

• Mainline lane shifts in advance of an intersection

• Mainline lane shifts in advance of a typical section change such as the addition of a mainline lane

• Mainline lane shifts in advance of a typical section change such as a change in median width

There are two methods by which an alignment transition or “shift” may be accomplished:

• The first method is to treat the transition or shift as though it were any other required alignment change. With this approach, a transition or shift would be accomplished through the use of a series of reverse curves. Quite often, the use of curve radii which do not require superelevation result in a length of transition greater than that required by providing a taper. Superelevation shall be utilized if warranted by normal procedures.

• The second method of accomplishing a transition or “shift” involves the use of tapers. Tapers are acceptable provided the following two conditions exist:
• The alignment shift is consistent with the cross slope of the roadway and does not require “shifting” over the top of an existing pavement crown
• The direction of the shift is not counter to the pavement cross-slope (from a superelevation or reverse-crown consideration)  

 

Taper lengths associated with shifts on Georgia roadways can be calculated as:

Case 1 - Design Speed ≥ 45 mph: 

L = W * s

Case 2 - Design Speed < 45 mph: 

             

 

Where: 

L = distance needed to develop widening (ft)

W = width of lane shift (ft)
s = design speed (mph)

 

Note: the Case 1 and Case 2 taper lengths described above are applicable to permanent conditions. For a more detailed discussion on temporary conditions associated with construction, refer to Chapter 12. Stage Construction.